A winterbourne is a stream or river that is dry through the summer months. A winterbourne is sometimes simply called a bourne, from the Anglo-Saxon for river, although this term can also be used for all-year water courses.

Winterbournes generally form in areas where there is chalk (or other porous rock) downland bordering clay valleys or vales. When it rains the chalk, which is porous, holds water in its aquifer, releasing the water slowly. During dry seasons the water table may fall below the level of the streams, causing them to dry out.

Exploitation of chalk aquifers as a domestic water source in Britain has had the effect of converting many streams and rivers into artificial winterbournes. This effect is controversial, and local campaigns have often been successful in reducing aquifer abstraction and reversing the effect. For an example, see the River Pang in Berkshire.